This invention relates to cryogenically cooled radiation detectors and more specifically to means in a cryostat for thermally and mechanically connecting a detector to a dewar.
Photon detectors or spectrometers are useful in measuring low levels of radionuclides. Examples of such measurements include monitoring of the environment and effluent discharge of nuclear power stations, measurement of food product for human intake and evaluation of the natural environment. Because of the radionuclide content of some samples, it has become standard practice to use the resolving power of germanium semiconductor detectors to identify and quantify the isotopes present. Germanium semiconductor gamma-ray spectrometers have been developed to provide optimum performance for such measurement. When x- or gamma rays impinge on a germanium detector, there is a finite probability of an interaction occurring which results in the creation of electron-hole pairs. If the impinging beam has an intensity, I, at a given energy, E, then it is absorbed in the detector according to an exponential law: EQU I(X)=I.sub.o.sup.(-X/.lambda.),
where I.sub.o is the beam intesity at the surface of the detector and X is the distance from the surface, and 1/.lambda. is the absorption coefficient and can be considered as the sum of three components due respectively to the photoelectric, Compton and pair-production processes. When an interaction occurs and electron-hole pairs are created, the electric field due to the bias voltage sweeps out the charge carriers resulting in an induced current pulse which is integrated at the input of a charge-sensitive preamplifier. Both charge carriers, the electrons and the holes, contribute to the current pulse. In order to provide for this operation, the germanium detector must be vacuum-jacketed and cryogenically cooled. Normally, such detectors are cooled to below 100.degree. K. A nominal operating temperature is 77.degree. K.
The germanium detector is incorporated in a cryostat. The cryostat comprises an evacuated housing surrounding the detector, the detector itself, a cryogenically cooled field effect transitor preamplifier in the housing, electronics circuit boards exterior to the housing mounted on support means coaxial with and extending from the detector, and means for communicating the detector with a cryogenic cooling source such as liquid nitrogen. Cryostats are provided in various sizes and configurations for various applications. Similarly, different dewars are utilized for varying applications. One form of cryostat may be connected to a dewar having a handle projecting therefrom which is balanced for maintaining the cryostat in a hand-held horizontal disposition. Different dewars may be provided to provide for eight hour or twenty-four hour holding time. Other dewars may be provided which have cryostats protecting from a top portion, a bottom portion or a side thereof. For different applications, nominal dewars may come in sizes of 0.4, 1.2, 7.5, 15, 25 or 30 liters. Different specific applications require different combinations of dewar and cryostat. The conventional means for connecting cooling devices to the detector has resulted in construction of a closure in which vacuum-jacket surrounding the detector is the same vacuum jacket that must surround the portion of the cryostat extending from the detector into the dewar and in some cases the dewar vacuum jacket. This widely successfully used and conventional design does not permit modularity of assembly between detector heads and different cryostats or different dewars. The experience in the art is that in reworking of detectors to move them from one cryostat assembly to another, yield rate is low. Consequently, an expensive inventory of cryostats must be maintained for a number of applications.